1. Field of the Invention
The present invention relates to a lower thread supplying apparatus for a sewing machine.
2. Description of the Related Art
Generally, a sewing machine includes a horizontally rotating hook for hitching an upper thread and an under thread so as to sew. The upper thread is inserted in a sewing needle vertically moving at an end of an arm part of the sewing machine. The under thread is extracted from a bobbin of a lower thread supplying apparatus under a needle plate which is positioned on the top surface of the sewing machine under the arm part.
At the beginning of a sewing operation, it is necessary to extract some extent of the lower thread from the bobbin for hitching with the upper thread. Then, the some extent of the extracted lower thread is required for securing in a space which is not affected by any external influence to prevent the generation of extra tension until the extracted thread is used up in the process of the sewing operation in order to properly form a seam.
With the above-mentioned conditions, in a conventional sewing machine, the lower thread supplying apparatus has an inner hook detachably accommodating the bobbin from an opening part arranged at the needle plate; a tension applying portion arranged in an internal hook to apply tension by hitching the lower thread extracted from the bobbin; a lower thread cutting knife positioned with a gap having predetermined length which is defined from the tension applying portion as required for the sewing operation; a lower thread holding space secured from the tension applying portion to the lower thread cutting knife to protect the lower thread from external influence; and a guide groove part to introduce the lower thread extracted from the bobbin into the lower thread holding space and to introduce the end portion of the thread to the lower thread cutting knife (for example, refer to Japanese Patent Examined Publication no. JP-B-2985256).
In the needle plate, a needle hole is formed for inserting a sewing needle, and the above-mentioned opening part is formed upstream in a cloth feeding direction and around the needle hole. Also, in the opening part, the lower thread cutting knife is attached at the same height with a top surface of the needle plate at a sewing operation so as not to hinder sewing cloths.
In addition, the lower thread cutting knife is arranged at a position downstream than the opening part in the cloth feeding direction and away from the needle hole in a direction perpendicular to the cloth feeding direction.
Further, the lower thread holding space and the guide groove are formed between the edge portion of the opening part and the lower thread cutting knife on the needle plate to follow along a line connecting between the bobbin and the lower thread cutting knife. In other words, the guide groove is formed on the top surface of the needle plate and the inner space of the groove is connected to the lower thread holding space.
After the lower thread is extracted from the bobbin to an upstream side of the cloth feeding direction and then hitched to the tension applying portion. After that, it is bent to a downstream of the cloth feeding direction and then inserted into the inner space from the starting end of the guide groove part at the edge of the opening part. Additionally, if the lower thread is pulled to a lower thread cutting knife side, the lower thread is moved along an edge of the top surface of the needle plate of the curved guide groove part, and the end of the lower thread is inserted into the lower thread supporting space and further introduced to the lower thread cutting knife and then the lower thread is cut.
At this time, the lower thread with its edge portion cut is secured with a length needed for further sewing operation as the lower thread cutting knife is positioned at a predetermined length away from the tension applying portion. Since a cover is mounted on the opening part, the lower thread between the tension applying portion and the starting edge of the guide groove part is placed under the cover foe protection from any external interference. Also, the lower thread between the starting edge of the guide groove part and the lower thread cutting knife is placed in the lower thread holding space for protection from any external influence.
Therefore, if the sewing operation is started, the lower thread is extracted smoothly without extra tension greater than that is imposed by the tension applying portion to properly form a seam from the beginning of the sewing operation.
As described above, the lower thread cutting knife is arranged at a position away with a predetermined distance from the tension applying portion to secure lower thread having a predetermined length. Thus, the guide groove part and the lower thread holding space are formed on the needle plate, at a lower thread passing route from the edge portion of the opening part to the lower thread cutting knife.
However, there has been a problem in that the reduction in strength of the needle plate occurs because the increase in the thin parts of the needle plate results from the presence of the guide groove part and lower thread holding space.
Further, it is preferable that the top surface of the needle plate is flat and smooth in view of the fact that cloth is placed and moved on the needle plate. However, there has been another problem in that the smooth movement of cloth is interfered to cause damage as the presence of the guide groove part imparts in unevenness on the top surface of the needle plate.